Effect of Cation Arrangement on the Polaron Formation and Colossal Permittivity in NiNb2O6
Localized polaron hopping is one of the common mechanisms of the colossal permittivity in oxides, however, the formation of which still remains to be explored. In this work, the effect of cation arrangement on the polaron formation and dielectric responses was experimentally revealed in two polymorphs of NiNb2O6, i.e. the ordered columbite (c-NiNb2O6) and disordered rutile (r-NiNb2O6) phases. Nb4+-type polarons and typical long-range conduction induced dielectric dispersion were observed in c-NiNb2O6 while Ni+-type polarons and colossal permittivity were found in r-NiNb2O6. A broad and temperature independent (in linewidth) EPR band at g ~ 1.982 was observed in c-NiNb2O6. By contrast, the r-NiNb2O6 exhibited coexistance of a broad band (g ~ 2.816) which showed the dipolar broadening effect, and a sharp band (g ~ 2.076) which was significantly enhanced at T ~ 580 K. The corelation between EPR band broadening and increased conductivity was also evidenced. The results suggest the nature of delocalizd Nb4+ polarons in ordered c-NiNb2O6 and that the Ni/Nb disordering facilitated the formation of localized Ni+ polaron which induced colossal permittivity in r-NiNb2O6.